Since plastic containers composed of a thermoplastic resin are excellent in, for example, lightness of weight, economical efficiency, shaping processability, impact resistance, transparency, reusability, they are used as containers for packaging foods and drinks more widely than conventional containers made of metal, glass, or the like. In particular, since laminates of a polyolefin resin and an ethylene-vinyl alcohol copolymer (hereinafter, this may be referred to as EVOH) or laminates of a polyolefin resin and a polyamide have low oxygen permeability and low water vapor permeability, they are widely used for, for example, containers of foods and drugs. Because there are increasing demands for long term storage performance, health and safety properties, and the like in foods and drinks in recent years due to the diversified lifestyles, awareness to safety, and the like of the consumers, containers made of multilayer structures having a layer of a barrier material are preferably used.
Further, plastic containers have increasing importance in containers to which high temperature processing for retort processing is applicable (hereinafter, may be simply referred to as “retort containers”) for the purpose of sterilizing and storing, or heating the contents in containers, particularly for foods and medical items.
However, a polyolefin resin layer and an EVOH or nylon layer have poor adhesion to each other. For this reason, they are usually laminated by carrying out a method of shaping by coextrusion in which, between each resin layer, an intermediate layer that adheres each layer is provided. As examples of adhesive resins to make such an intermediate layer, polyolefin resins according to Patent Documents 2 and 3 are well known that are graft modified with an unsaturated carboxylic acid or derivative thereof. As a material of same kind, an olefin-unsaturated carboxylic acid copolymer with an unsaturated carboxylic acid or derivative thereof in the main chain is also known.
In addition, as known methods to improve the adhesion strength for polar resins, Patent Document 4 discloses a method of further adding a low density polyethylene to a graft modified polyolefin resin, Patent Document 5 discloses a method of adding a hydrocarbon-based elastomer, and Patent Document 6 discloses a method of melt-kneading a mixture made of a propylene-ethylene block copolymer, an ethylene-α-olefin random copolymer, an unsaturated carboxylic acid or derivative thereof, and an organic peroxide.
However, in such an adhesive resin and such a multilayer structure, carrying out retort processing may decrease the adhesive strength between an intermediate layer comprising an adhesive resin and a barrier resin layer, which may result in peeling. Moreover, the preliminary use of an adhesive resin having high adhesion may result in occurrence of continuous irregularities in the interface of each interlayer, which may be referred to as “orange peel”, when producing laminates by, for example, coextrusion and thus the appearance sometimes deteriorate. Even when using, as an adhesive layer, the graft modified polyolefin resins according to Patent Documents 1 to 3 and a composition in which a third component is added to the graft modified polyolefin resins according to Patent Document 4 to 6, the performance is insufficient for any of the applications at normal temperature, applications of boiling processing, and applications of retort processing.
Patent Document 2: JP 51-098784A
Patent Document 3: JP 52-026548A
Patent Document 4: JP 54-040113B
Patent Document 5: JP 54-040112B
Patent Document 6: JP 56-041205A